There are two general types of ink jet printing, drop-on-demand (DOD) and continuous (CIJ). Drop-on-demand printing, as its name suggests, produces droplets of ink as and when required in order to print on a substrate. Continuous ink jet printing, to which the present invention relates requires a continuous stream of ink which is broken up into droplets which are then selectively charged; either charged or non-charged droplets are allowed to pass to a substrate for printing, charged droplets being deflected in an electric field either on to the substrate or into a gutter (according to design) where the non-printed droplets are collected for re-use. In the first case, the droplets are deflected by an electric field onto the substrate with the uncharged drops going straight on to be collected in a gutter for re-use. The amount of charge also determines the relative printed position of the drops. In the second case, the droplets are deflected into an offset gutter, with the printing drops being the uncharged ones going straight onto the substrate. The obvious advantage of printing with the uncharged drops is that, in a multi-jet printer where several drop generators are aligned perpendicular to a moving substrate, the alignment of the drops printed on the substrate is not dependent on the ability to accurately and uniformly charge the drops. As long as the charge on the droplets is sufficient for the drops to be deflected into the gutter aperture, small variations in the charge applied will not affect the quality of the resulting print. This second type of printer is generally known as a binary jet printer as the droplets are either charged or uncharged (and do not intentionally carry varying amounts of charge that determine print position).
In typical continuous ink jet printers the printhead has a droplet generator which creates a stream of droplets of ink by applying a pressure modulation waveform to the ink in a cavity in the printhead and the continuous ink stream leaving the printhead breaks up into individual droplets accordingly. This modulation waveform is usually a sinusoidal electrical signal of fixed wavelength. The stream of ink leaving the printhead breaks up into individual drops at a distance (or time) from the printhead commonly known as the break-up point, that is dependent on a n umber of parameters such as ink viscosity, velocity and temperature. Provided these and other factors are kept relatively constant, then a given modulation waveform will produce a consistent break-up length. In order to induce a charge on the droplet, the charging waveform must be applied to the stream at the moment before the drop separates from the stream, and held until the drop is free (ie. must straddle the break-up point). It is therefore necessary to know the phase relationship between the modulating waveform and the actual drop separating from the stream (ie. during which part of the sinusoidal modulation waveform does break-up occur).
One method of determining this phase relationship involves a charge detector (and associated electronics), position somewhere after the charging electrode, which can detect which drops have been successfully charged. A half width charging pulse, progressively advanced by known intervals relative to the modulation waveform, is used to attempt to charge the droplets and the detector output analysed to determine correct charging. Because of the half width pulse, theoretically half the tests should pass and half should fail. The full width pulses used for printing would then be positioned to straddle the detected break-up point. The number of intervals that the waveform is divided into, and therefore the number of possible different phases, can vary from system to system, but usually the timing is derived from a common digital close signal, and therefore is usually a binary power (ie. could be 2, 4, 8, 16, 32 etc.). Typically, 2 and 4 intervals would not give sufficient resolution, and 32 intervals upwards would make the tests too time consuming. Using 16 intervals (ie. 16 different phases) is considered to give more than adequate accuracy without involving a detrimental number of tests.
In a multi-jet print, due to manufacturing tolerances of the nozzles and the characteristics of the (usually common) ink cavity, the break-up point for each of the streams, and therefore the phase setting for printing will be different.
Modern multi-jet printers, in order to be able to print high-quality graphics and true-type scalable fonts, utilise a large number of ink streams, placed very closely together (typically 128 jets at a spacing of 200 microns).
Although it has proved possible to manufacture charge electrodes at the required spacing, to individually charge the streams, it would not be practical to duplicate existing charge electrode driver circuitry 128 times, and so current trends lean towards the use of an integrated driver solution in which a large number of the drive circuits are implemented in one Integrated Circuit device, in order to save space, reduce power etc. With such a device, for practical reasons, it is not possible to enable, or set the level of charging voltage on an individual jet basis, and so all the high voltage drivers within the device have a common enable and common power supply.
Additionally, at present it is not possible to have a separate phase detector for each stream. The probability is that the individual detectors would never be able to isolate the charge from their own stream from the effects of any adjacent streams.
As a final handicap to existing phasing methods being applied to this type of printer, it must be noted that the "normal" condition for the drop stream, ie. not printing, is for all the drops to be charged. Therefore, to test individual jets would require the detection of the non-charged state, resulting in ink being sent to the substrate. Also, the phase detector circuitry would more than likely not be able to distinguish the change in charge passing the detector when a single jet was turned off, against a background of 127 jets still on.
Therefore conventional phase detection methods are not suitable for modern high-resolution binary inkjet printers.
The present invention is directed towards overcoming the above problems.